Not until the comment section will you find reflection on time you can actually dedicate to language acquisition. Even on unconscious level, adults are generally already far more overloaded with many other topics to deal with.
So yes, obviously if you don't have time as an adult to learn a language, you won't learn it as well as if you can dedicate the rest of your life to it. But the point of the results of "critical period" studies is that even if you dedicate the rest of your life to it, you'll still be at a disadvantage compared to a child, when it comes to attaining perfect phonetic and linguistic fluency.
I do wonder if there's some limit on the number of phonemes you can learn, and how many hours are required to "solidify" a phoneme. But I definitely think the idea of a "unity course" is a good one.
Personally I had a French "class" in elementary school, where I barely learned anything - I certainly can't speak French - but I can pronounce French words better than I can pronounce any Spanish words (despite three years of Spanish class in college). Those French classes were basically just learning to count to 100 and singing songs like "alouette."
It took me 3 years of about 2 hours a day in high school to learn approximately 6000 words in English, my native language. This was in addition to my schooling where I was writing essays and book reports pretty much weekly. Probably 10+ hours per week for history and english. Plus all the interactions in my native language.
That's a LOT of language practice. That's almost 1800 hours on vocabulary alone. Add on 1500 hours for essays. That's approaching 3200 hours which strongly exceeds the amount of time required to gain general proficiency in a Category V language (2200 hours). That's almost 10+ hours per week on top of school hours themselves.
How many people ever put in that amount of effort into a second language? How many people have the time to put in that amount of effort into a second language?
While most people don't have time to put that many hours into studying a second language outside of a country where it's the native tongue, many immigrants have no choice but to immerse themselves in a new language just by the default behavior of living their life in a new country that doesn't speak their first language.
Yes, such immigrants will always be a few thousand hours behind people who started in America, but surely there's some plateau where the improvements become marginal. I don't think the critical period advantage is about the early volume of hours so much as the age period when they're spent - i.e. if you compare someone who moves at 20 vs. 40, the results will be the same, but both will be worse than someone who moved at 8.
As a native English speaker, I'm not sure how many English words I know, nor how I would even start counting! However, I am currently learning German, and seem to have found a comfortable pace of learning about twenty words each day, studying for half an hour each day. This doesn't include general exposure to the words through other German practice. If it is possible to keep that up long-term, that means that 6000 German words would take just under a year for me to learn in the ideal case (probably a bit longer, as so-called 'strong' words have multiple irregular forms that are effectively different words).
Mostly SAT prep. It took my verbal score from 550 to 780 (out of 800).
However, extra vocabulary is also one of the fastest ways to improve your writing. My writing got a lot more concise the more vocabulary I commanded. Essays that used to take 20 hours+ and maybe 6-8 pages were now 4-5 hours and 2-3 pages.
And, once you do this, you find out how very few words we actually use on a daily basis.
“The difference between the almost right word and the right word is really a large matter. ’tis the difference between the lightning bug and the lightning.” -- Mark Twain
Something about that doesn't feel right, so I hope you will be able to explain if I am missing something. However, were it to be true, it sounds like the best way of 'futureproofing' children's early language education so that they can learn languages later in life is to bring them up to speak, at least at a basic level, a set of languages that covers a large proportion if not all of the distinct phonemes of all languages.
I'd be curious to see a visualization of the overlapping phonemes between languages, like a large Venn diagram kinda thing (or maybe just a graph). I think this data is probably available and a visualization wouldn't be too difficult to create - maybe it already exists but I couldn't find a comprehensive one with a brief search.
Besides tsu and dzu, every phoneme in Japanese is accounted for in the Spanish language. I believe that the Spanish phonemes are a nearly perfect subset of English phonemes.